Uhhh... no, I didn't suggest "bigger res screen"? I suggested setting a custom resolution greater than his display's native resolution (for example 1440p on a 1080p native display) and then downsampling. Why would I suggest he get a 4K screen and then downsample? That makes no sense. What would he even downsample from? 8K, which no GPU currently on the market could push at playable frame rates?

MA620P and MA410M are both too tall for your case, which has a maximum cooler height (according to the manufacturer website) of 155mm. If you want a dual-tower cooler like the MA620P with a lower profile, check out the Scythe Fuma Rev B.

Maybe fine detail wasn't the best way to describe it, but it helps with aliasing in the same way an actual higher resolution makes AA less necessary. Personally I think resolutions above 1080p on a 24" screen are pretty pointless so I can only guess about what he's trying to achieve.
I fully agree that actual 4K/144 would be the ideal for him if he could afford it, but I don't know why you're telling me so as I didn't suggest he get a 4K/144 display.

You didn't do anything wrong. @Stormseeker9 is incorrect. There is a radio button to buy a used model on the page but you linked to the listing for a brand new one.
And that is a good display, I used to own one myself, the only downfall of it is that the FreeSync range tops out at 90hz unless you fiddle with CRU.